The present document is related to and claims priority on Japanese Priority Documents 11-370,457, filed on Dec. 27, 1999, and 2000-176,691, filed on Jun. 13, 2000, the contents of both of which are hereby incorporated herein by reference.
1. Field of the Invention
The present invention relates to an arrayed waveguide grating type optical multiplexer/demultiplexer used as an optical multiplexer/demultiplexer in, for example, wavelength division multiplexing optical communications, and to a method of manufacturing the same.
2. Discussion of the Background
Recently, in optical communications research and development of optical wavelength division multiplexing communications has actively been pursued as a way to exponentially increase transmission volume, and the results are being put into practice. Optical wavelength division multiplexing communications uses, for example, a technique of division multiplexing a plurality of light beams each having a different wavelength from one another to transmit them. For systems using such optical wavelength division multiplexing communications, a light transmissive device or the like is provided to enable the receiver of the optical communication to take out light beams separately on the basis of their wavelengths from the transmitted light beams that have undergone the wavelength division multiplexing. The light transmissive device only transmits light of certain given wavelengths.
Examples of the light transmissive device include an arrayed waveguide grating (AWG) including a planar lightwave circuit (PLC) such as shown in FIG. 15. The arrayed waveguide grating has a waveguide forming region 10 formed from quartz-based glass on a substrate 1 made of silicon or the like. The waveguide forming region 10 has a waveguide structure as illustrated in FIG. 15 and formed from a core.
The waveguide structure of the arrayed waveguide grating includes one or more optical input waveguides 2 arranged side by side, a first slab waveguide 3 connected to the output ends of the optical input waveguides 2, an arrayed waveguide 4 connected to the output end of the first slab waveguide 3, a second slab waveguide 5 connected to the output end of the arrayed waveguide 3, and a plurality of optical output waveguides 6 arranged side by side and connected to the output end of the second slab waveguide 5. The size of the arrayed waveguide grating can be set, for example, such that A=B=40 mm.
The arrayed waveguide 4 propagates light output from the first slab waveguide 3, and includes a plurality of channel waveguides 4a arranged side by side. Lengths of adjacent channel waveguides 4a are different from each other with the differences (xcex94L) preset. The number of optical output waveguides 6 is determined, for example, in accordance with how many light beams having different wavelengths from one another are to be created as a result of demultiplexing or multiplexing of signal light by the arrayed waveguide grating. The channel waveguides 4a constituting the arrayed waveguide 4 are usually provided in a large number, for example 100. However, FIG. 15 is simplified and the number of the channel waveguides 4a, the optical output waveguides 6, and the optical input waveguides 2 in FIG. 15 does not reflect the actual number thereof.
The optical input waveguides 2 are connected to, for example, transmission side optical fibers (not shown), so that light having undergone the wavelength division multiplexing is introduced to the optical input waveguides 2. The light output from the optical input waveguides 2 is introduced to the first slab waveguide 3, is diffracted by the diffraction effect thereof, and enters the arrayed waveguide 4 to travel along the arrayed waveguide 4.
After traveling through the arrayed waveguide 4, the light reaches the second slab waveguide 5 and then is condensed in the optical output waveguides 6 to be output therefrom. Because of the preset differences in lengths between adjacent channel waveguides 4a of the arrayed waveguides 4, light beams after traveling through the arrayed waveguides 4 have different phases from one another. The phase front of many light beams from the arrayed waveguide 4 is tilted in accordance with the differences and the position where the light is condensed is determined by the angle of this tilt.
Therefore, light beams having different wavelengths are condensed at different positions from one another. By forming the optical output waveguides 6 at these positions, light beams xcex1, xcex2, . . . xcexn having different wavelengths can be output from the respective optical output waveguides 6 provided for the respective wavelengths.
In other words, the arrayed waveguide grating has an optical multiplexing/demultiplexing function. With this function, the arrayed waveguide grating can demultiplex light input from the optical input waveguides 2, which has previously undergone the division multiplexing and possesses different wavelengths from one another, into light beams of one or more wavelengths, and then output the light beams from their respective optical output waveguides 6. The central wavelength of light to be demultiplexed is in proportion to the differences (xcex94L) in lengths of adjacent channel waveguides 4a constituting the arrayed waveguide 4 and to the effective refractive index ne of the channel waveguides 4a. 
Having the characteristics as above, the arrayed waveguide grating can be used as a light transmissive device for optical multiplexing/demultiplexing applied to a wavelength division multiplexing transmission system. For instance, as shown in FIG. 15, light beams which have undergone wavelength division multiplexing and having wavelengths of xcex1, xcex2, xcex3, . . . xcexn (n is an integer equal to or larger than 2), respectively, are input to one of the optical input waveguides 2. The light beams are diffracted in the first slab waveguides 3, reach the arrayed waveguides 4, and travel through the arrayed waveguides 4 and the second slab waveguides 5. Then, as described above, the light beams are respectively condensed at different positions determined by their wavelengths, enter different optical output waveguides 6, travel along their respective optical output waveguides 6, and are output from the output ends of the optical output waveguides 6.
The light beams having different wavelengths can then be further taken out through optical fibers for outputting light (not shown) that are connected to the output ends of the optical output waveguides 6. When connecting the optical fibers to the optical output waveguides 6 and to the optical input waveguides 2, an optical fiber array is prepared for each. In the optical fiber array, connection terminal faces of the optical fibers are arranged and fixed into a one-dimensional array. The optical fiber array is fixed to the connection terminal faces of the optical output waveguides 6 or to the optical input waveguides 2 to thereby connect the optical fibers to the optical output waveguides 6 or to the optical input waveguides 2.
The above arrayed waveguide grating has such light transmission characteristics (wavelength characteristics of transmission light intensity in the arrayed waveguide grating) of light beams output from the optical output waveguides 6 such that with the respective light transmission central wavelengths (e.g., xcex1, xcex2, xcex3, . . . xcexn) as the center, the light transmittance of the output light beams becomes smaller as the wavelength deviates from their respective light transmission central wavelength.
Every light transmission central wavelength xcexo is determined by the effective refractive index ne of the arrayed waveguide 4, the difference (xcex94L) in length of adjacent channel waveguides 4a of the arrayed waveguide 4, and diffraction order m, and is expressed by the following numerical expression (1).
xcexo=nexc2x7xcex94L/mxe2x80x83xe2x80x83(1)
Therefore, the wavelength indicative of the light transmission characteristics with regard to one of the optical output waveguide 6 is not always one, but there may be plural central wavelengths depending on the diffraction order set thereto. It is possible to demultiplex light into a plurality of optical signals having a certain wavelength interval xcex94xcex (nm) with the light transmission central wavelength xcexo as the center. Accordingly, only the central wavelength xcexo is considered in the discussion below.
The arrayed waveguide grating utilizes the principle of reciprocity (reversibility) of an optical circuit and, hence, has the function of an optical multiplexer as well as the function of an optical demultiplexer. That is, in a manner reverse to that already discussed with respect to FIG. 15, a plurality of light beams having different wavelengths from one another may be input to respective optical output waveguides 6. The input light beams travel along propagation routes opposite to the routes discussed above with respect to FIG. 15, are multiplexed in the arrayed waveguide 4 and in the first slab waveguide 3, and then are output from one of the optical input waveguides 2.
In such an arrayed waveguide grating as mentioned above, the wavelength resolution of the grating is in proportion to the difference in lengths (xcex94L) between the channel waveguides 4a of the arrayed waveguides 4, which are one of the components of the grating. When the arrayed waveguide grating is designed to have a large xcex94L, it is theoretically possible to multiplex/demultiplex light to accomplish wavelength division multiplexing with a narrow wavelength interval. It is thus theoretically possible for the arrayed waveguide grating to have a function of multiplexing/demultiplexing a plurality of signal light beams, specifically, a function of demultiplexing or multiplexing a plurality of optical signals with a wavelength interval of 1 nm or less, which is a function deemed necessary for optical wavelength division multiplexing communications of high density.
To manufacture an arrayed waveguide grating as discussed above, for example, first flame hydrolysis deposition is used to form an under cladding layer and a core layer on a silicon substrate, then a photomask is prepared on which the waveguide structure of the arrayed waveguide grating is drawn, a transfer is performed by photolithography through the photomask, the arrayed waveguide grating pattern is transferred onto the core layer by reactive ion etching, and then flame hydrolysis deposition is again used to form an over cladding layer. The arrayed waveguide grating is thus manufactured.
The arrayed waveguide grating of FIG. 15 is conventionally formed with a quartz-based glass material as a main component, and due to temperature dependency of this quartz-based glass material, the light transmission central wavelength xcexo of the arrayed waveguide grating shifts depending on the temperature. This temperature dependency extends to so great a degree that, for instance, when the change in temperature is 50xc2x0 C. or more in an arrayed waveguide grating designed and manufactured using setting values generally used in the background art, the light transmission central wavelength shifts by 0.5 nm or more. The value 0.5 nm is fatal to an arrayed waveguide grating desired to demultiplex or multiplex light with a very narrow wavelength interval of 1 nm or less.
The present inventors therefore believe that there is a great importance in realizing an arrayed waveguide grating type optical multiplexer/demultiplexer that can control the temperature dependency of the light transmission central wavelength. According to the view of the present inventors, easiness in manufacturing the arrayed waveguide grating type optical multiplexer/demultiplexer and smallness of insertion loss are also objects significant in putting into practice an arrayed waveguide grating type optical multiplexer/demultiplexer as a device for wavelength division multiplexing communications.
The present invention has been made in order to address the problems noted above, and an object of the present invention is therefore to provide an arrayed waveguide grating type optical multiplexer/demultiplexer that is easy to manufacture, can reduce temperature dependency of a light transmission central wavelength, and can reduce insertion loss, and to provide a method of manufacturing the same.
To achieve the above and other objects, in an arrayed waveguide grating type optical multiplexer/demultiplexer of the present invention, a slab waveguide is divided into two by intersecting planes that intersect the route of the light traveling along the slab waveguide. The intersecting planes serve as dividing planes and divide a waveguide forming region into a first waveguide forming region that includes one portion of the divided slab waveguide and a second waveguide forming region that includes the other portion of the divided slab waveguide. One or both of the first waveguide forming region and the second waveguide forming region are moved along the dividing planes by a position shifting member. Therefore it is possible to compensate, with the use of the movement by the position shifting member, shifts in light transmission central wavelengths of the arrayed waveguide grating which is caused by, for example, the temperature change of the arrayed waveguide grating.
In the arrayed waveguide grating type optical multiplexer/demultiplexer according to the present invention, when the position shifting member is arranged such that its one end is secured on the first waveguide forming region and its other end is secured on the second waveguide forming region, the structure of the device is simplified and precision is improved. Furthermore, the cost of the device is reduced and the yield thereof is increased.
Further to achieve the above and other objects, in the method of manufacturing an arrayed waveguide grating type optical multiplexer/demultiplexer of the present invention, at least one slab waveguide is divided into two by intersecting planes that intersect the route of light traveling along the slab waveguide. The waveguide forming region is divided by the dividing planes into the first waveguide forming region that includes one portion of the divided slab waveguide and a second waveguide forming region that includes the other portion of the divided slab waveguide. A position shifting member with a function of moving one or both of the first and second waveguide forming regions along the dividing planes is fixed before the division such that the position shifting member secures its one end on the first waveguide forming region and secures its other end on the second waveguide forming region. Therefore, the relative positions of the first waveguide forming region and the second waveguide forming region before the division are almost the same as those after the division.